Optical fibers are in widespread use today as transmission media because of their large bandwidth capabilities and small size. Developments in the optical fiber communications field have been rapid. However, the technology still is undergoing major shifts in direction. An example of a shift which is driven by demand for higher bandwidths is that from multimode to single mode fibers.
Although desired for their large bandwidth capabilities and small size, light-transmitting optical fibers are mechanically fragile, exhibiting low-strain fracture under tensile loading and degraded light transmission when bent. The degradation in transmission which results from bending is known as microbending loss. As a result, cable structures have been developed to protect mechanically the optical fibers in various environments. For example, a cable for use in a duct must be capable of withstanding tensile loads applied when the cable is pulled into the duct and stresses caused by tortuous or arcuate paths.
Cable structures which have been developed for optical fibers include loose tube, stranded and ribbon cables. For a description of loose tube cables, see, for example, D. Lawrence and P. Bark "Recent Developments in Mini-Unit Cable" published at pp. 301-307 of the Proceedings of the 32nd International Wire and Cable Symposium, 1983. See also U.S. Pat. No. 4,153,332. In some situations, especially duct systems which include many bends such as those in loop plant in urban areas, relatively high tensile loads are expected.
In one type of optical communications cable, a plurality of optical fibers is enclosed in an extruded plastic tube to form a unit and a plurality of these tubed units is enclosed in a common extruded plastic tube which is enclosed in a sheath system. Each unit is made on a manufacturing line and inventoried until it is stranded with other units on another line whereat a plastic jacket also is applied.
What still was sought was a cable for optical fiber transmission which departed from the stranding of units and which inhibited the introduction of undue stresses that could lead to microbending losses in the optical fibers. A cable which satisfies these needs is disclosed in App. Ser. No. 721,533 which was filed on Apr. 10, 1985, now pending, in the names of C. H. Gartside, III, A. J. Panuska, and P. D. Patel. That cable includes a plurality of optical fibers which are assembled together in a core without intended stranding to form units which extend in a direction along a longitudinal axis of the cable. A length of tubing which is made of a plastic material encloses the plurality of units and is parallel to the longitudinal axis of the cable. The ratio of the cross-sectional area of the plurality of optical fibers to the cross-sectional area within the tubing does not exceed a predetermined value.
A sheath system for the just-described cable may be one disclosed in U.S. Pat. No. 4,241,979 which issued on Dec. 30, 1980 in the names of P. F. Gagen and M. R. Santana. A bedding layer, about which strength members are wrapped helically, is added between plastic extruded inner and outer jackets to control the extent to which the strength members are encapsulated by the outer jacket. The cable includes two separate layers of metallic strength members, which are wrapped helically in opposite directions. Under a sustained tensile load, these two layers of strength members produce equal but oppositely directed torques about the cable to insure the absence of twisting. Advantageously, the strength members not only provide the necessary strength characteristics for the cable, but also reinforce the sheath and help protect the optical fiber from external influences.
Such a sheath system as described may be replaced with one in which only one layer of metallic strength members is used. See Appl. Ser. No. 825,291 filed on Jan. 31, 1986 in the names of W. D Bohannon, Jr., et al, now U.S. Pat. No. 4,765,712. A core is enclosed by a tube which is made of a plastic material, a shield system and an outer plastic jacket. The shield system provides rodent and/or lightning protection. Strength is provided by a plurality of helically wrapped members which are disposed in a single layer concentric with the core and which in a preferred embodiment are adjacent to an outer surface of the shield with substantial portions of their peripheries embedded in the plastic of the outer jacket.
In some prior art cables, the metallic wires of the sheath system in hereinbefore-identified U.S. Pat. No. 4,241,979 have been replaced with glass fiber members at least some of which are capable of withstanding expected compressive as well as tensile loading. Compressive loading occurs when the cable tends to contract during initial shrinkage of the jacket material, during bending, and during thermal cycling. Interposed between a tubular member which encloses a core and a plastic jacket is a layer of strength members which are wrapped helically about the core. A first plurality of the strength members are relatively flexible and a second plurality of the strength members have sufficient compressive stiffness and are coupled sufficiently to the jacket to provide a composite arrangement which is effective to inhibit contraction of the cable.
Although the sheath systems of U.S. Pat. No. 4,241,979 and in Appl. Ser. No. 825,291 meet the aforementioned needs, efforts have continued to find alternatives. The number of strength members in prior art cables is usually high, and core entry necessitates the violation of the integrity of these strength members. Further, these strength members generally are wound helically about the core which process involves the rotation of relatively heavy supplies.
What is still needed and what seemingly is not provided by the prior art is a cable having a compact and relatively uncomplicated sheath system which is capable of withstanding compressive as well as tensile loading. The sought-after cable should be adaptable to a variety of environments and accommodate a plurality of optical fibers. Also, the sought-after cable should be one which may be made without involving the rotation of relatively heavy supplies and one in which the core may be accessed without violating the integrity of the strength member system.